Compressive strength is the measure of the greatest compressive loading concrete can withstand. The compressive strength test is completed under the standard procedure of ASTM as ASTM C39/C39M27 for cylindrical specimens having standard dimensions as 150 mm diameter and 300 mm length.
Figure 3 displays the outcomes of the compressive strength test on concrete cylinders with different dosages. The compressive strength of concrete having coconut shell aggregate, silica fume, and glass fibers is increased up to a certain level but then decreased as shown in Fig. 3. Standard deviation and coefficient of variation for compressive strength at 7 and 28 days is presented in Table 7. From the outcome, it can be understood that concrete with 45% CS aggregate, 1.5% glass fibers, and 15% silica fume, showed a much better result than all specimens in terms of compressive strength. Although it is revealed that excess content of coconut shells reduce strength due to the poor bond between cement and coconut shell8. The positive response of compressive strength is because of the pozzolanic reaction of SiO2 in silica fume with CH of cement producing additional cementitious compounds. It has been also reported that compressive strength considerably improved with silica fume28. The extra binder formed by the silica fume reaction with existing lime Ca (OH2) permits silica fume concrete to continue to increase strength over time. However, at a higher dosage of silica fume (beyond 15% by weight of cement) strength reduce due dilution effect which leads to alkali-silica reaction due to a higher quantity of unreactive silica available is because of the silica fume high quantity. Furthermore, the positive effect on compressive strength is due to the confinement of the fiber reinforcement on the concrete sample. Compression produces an expansion laterally, and with it, tension and shear. The tension and shear are resisted by the fibers. Therefore, compression is increased. When the percentage of fibers is more this confinement can decrease concrete sample transversal deformation and enhance its compressive strength.
Figure 3Compressive strength of concrete at 7 and 28 days of curing.
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Table 7 Standard deviation and coeffiecient of variation of compressive strength results (MPa).Full size table
A relative analysis was carried out in which the curing of 28 days age of control mix compressive strength was considered the related mix, and from this, different blends with changing percentages are compared, as shown in Fig. 4. At 7 days of curing, compressive strength was about 23% less than that of the control (28 days) at 45% CS aggregate, 1.5% glass fibers, and 15% silica fume (optimum dosage). Compared to the control, 45% CS aggregate, 1.5% glass fibers, and 15% silica fume (optimum dosage) showed 20% higher compressive strength at 28 days of curing. Therefore, it is recommended to use 45% CS aggregate, 1.5% glass fibers, and 15% silica fume (optimum dosage) in concrete to make concrete with good compressive strength.
Figure 4Relative analysis of compressive strength.
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It is a technique of evaluating the concrete tensile strength using a cylinder. According to ASTM C496-7129, a split cylinder test was completed on cylindrical samples of 300 mm height and 150 mm diameter and at the ages of 7, and 28 days of curing.
Figure 5 presents the split tensile strength of different mixes while its standard deviation and coefficient of variation for 7 and 28 days is displayed in Table 8. Adding glass fibers to concrete impressively enhances the concrete flexural and split tensile attributes during the hardened stage, for example, rigidity, flexural strength, toughness, and flexibility30. Past examinations have demonstrated that the consideration of fibers fundamentally enhances the split tensile strength of lightweight aggregate concrete31,32. In the present examination, the addition of glass fibers and silica fume to coconut Shell concrete applies a gainful impact on split tensile and flexural strength. After the test, it was noted that concrete achieved its highest flexural and split tensile strength at 45% coconut shell aggregate, 1.5% glass fibers, and 15% silica fume. Although it has been revealed that surplus quantity of coconut shell aggregates reduces strength due to poor bond between cement paste resulting in porous concrete13. Increasing the percentages of the above materials reduced the concrete split and tensile flexural strength. Fibers are mixed in concrete to increase the flexibility of concrete by halting the onset of tension cracks or preventing the generation of cracks in such a manner that the tensile strength of (SFRC) steel fiber reinforced concrete displays better conduct than normal concrete. Fibers assists in dispensing the applied forces to the whole body of concrete. Fibers are known to enhance the tensile capacity of post-cracking behavior21,33. Fibers have shown more substantial effects on split tensile and flexural strength at 0.5 to 2.0 percent volume fractions added in the study21,34. Moreover, Silica fume particles are 100 times smaller than cement grains and so they can pack very well with cement grains. They also react with CH to form CSH which gives additional binding properties and results in strength increase. However, at higher dosage Mix 4 (60% CS aggregate, 2.0% glass fibers, and 20% silica fume), the workability of concrete decreases, because the size of silica fume is so small, when we add more silica fume it will increase the surface area and demand more water and if we add more water it will reduce the strength.
Figure 5Splitting tensile strength of concrete.
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Table 8 Standard deviation and coeffiecient of variation of split tensile strength results (MPa).Full size table
A relative analysis was carried out in which the curing age of the 28-days control sample split tensile strength was considered the related mix, and from this, different blends with changing percentages are compared, as shown in Fig. 6. At 7 days of curing, split tensile strength was about 14% less than that of the control (28 days) at 45% CS aggregate, 1.5% glass fibers, and 15% silica fume (optimum dosage). Compared to the control, 45% CS aggregate, 1.5% glass fibers, and 15% silica fume (optimum dosage) showed 22% higher spilt tensile strength at the curing age of 28 days. Therefore, to make concrete with good split tensile strength it is recommended to used 45% CS aggregate, 1.5% glass fibers, and 15% silica fume (optimum dosage) in concrete.
Figure 6Relative analysis of splitting tensile strength.
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Comparison of predicted values with experimental tensile strength values utilizing ACI-318.11 codes is displayed in Fig. 6. Equation (1) can be used to predict the values of split tensile strength from compressive strength.
$$ f{\text{sp}} = {\text{~}}0.53 \times {\text{~}}\sqrt {f{\text{c}}} $$
(1)
It is observed that entire empirical values locate well in anticipated values utilizing ACI-318.11 codes. Regression models between experimental values of split tensile strength and compressive strength are displayed in Fig. 7. A strong correlation occurs (R2 > 0.94) amid both strengths.
Figure 7Co-relation between splitting tensile strength and compressive strength.
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Density is an indirect method to determine the durability of concrete i.e. higher density gives more dense concrete leading to fewer voids resulting in more durable concrete. Concrete density with varying percentages of different dosages was determined as per ASTM C13835.
Figure 8 shows the density of concrete with different dosages. It can be observed that concrete density increases up to Mix 3 (45% CS aggregate, 1.5% glass fibers, and 15% silica fume) and then decreases gradually having a maximum density at Mix 3 (45% CS aggregate, 1.5% glass fibers, and 15% silica fume) while minimum density is obtained at Mix 4 (60% CS aggregate, 2.0% glass fibers, and 20% silica fume). Although it has been reported that excess amount of coconut shell aggregates reduces strength due to poor bond between cement paste resulting in porous concrete. Fibers controls and constrain the development of cracks in concrete caused both in the plastic and hardened stage of concrete hence confirming a more durable concrete30,31,33. Furthermore, silica fume enhances density due to pozzolanic reaction i.e. giving secondary C–S–H gel which rises the paste viscosity leading to more packed concrete. However, mix 4 (60% CS aggregate, 2.0% glass fibers, and 20% silica fume), shows less density than the control specimen. The density of coconut shell aggregate is less than coarse aggregate, and if we add more quantity of coconut shell aggregate it will lead to concrete with less strength than the control specimen.
Figure 8Density of concrete.
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As density directly affects strength. Higher density gives more dense concrete leading to less void which ultimately increases strength. Therefore, a strong correlation is existing between density and compressive strength. The correlation between compressive strength and density is displayed in Fig. 9. It can be observed that the regression line between compressive strength and density is appeared to be linear. Regression modal shows a strong co-relation between compressive strength and flexure strength having an R2 value greater than 90%.
Figure 9Co-relation between density and compressive strength.
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It is a non-destructive test performed to evaluate concrete integrity and homogeneity as per ASTM C 597-0232. Subjective evaluation of concrete strength and degree of concrete in various areas of members of structure might be accomplished by utilizing this technique. Any depth of surface cracks, concrete cover examination, lack of coherence in the cross-section (for example cracks can additionally be determined). UPV test of all mixes was estimated, and, the relationship between the UPV and compressive strength of CS concrete was calculated at 28 days as shown in Fig. 10. Concrete is sound an 'acceptable' state when its ultrasonic pulse velocity values are in the middle of 3.70 km/s and 4.61 km/s36. Whole concrete samples were exposed to the Ultrasonic Pulse Velocity test earlier than the destructive tests. Overall, the values of the ultrasonic pulse velocity Test of entire blends improved with rising compressive strength. The values of the ultrasonic pulse velocity test entire blends are between 3.76 and 4.21 km/s. So, it can be observed from the test, that the coconut shell concrete with glass fibers have more ultrasonic pulse velocity values.
Figure 10UPV test results.
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Ultrasonic pulse velocity calculations corresponding to their related compressive strength are shown in Fig. 11. Equation (2) can be used to predict values of ultrasonic pulse velocity from compressive strength.
$$ f{\text{sp}} = {\text{~}}fck = 0.{\text{13}}v^{{{\text{4}}.0{\text{481}}}} $$
(2)
Figure 11Co-relation between UPV and compressive strength.
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fck represents compressive strength in (Mpa), and v is the UPV (km/s).
Regression models between Ultrasonic Pulse Velocity and experimental values of compressive strength show a strong correlation having R2 greater than 90 percent.
Water absorption is an indirect measurement of concrete durability. Mostly harmful chemicals are present in water. These chemicals react with concrete ingredients, which changes the properties of concrete. Extra water present in the pores of concrete results in freezing and thawing cycles effect because of the change in temperature, which results in a concrete crack. Therefore, a water absorption test was conducted on all samples at 7 and 28 days.
Water absorption test results are displayed in Fig. 12. A general trend indicates that water absorption decreases up to Mix 3 (45% CS aggregate, 1.5% glass fibers, and 15% silica fume) and then increases gradually at Mix 4 (60% CS aggregate, 2.0% glass fibers, and 20% silica fume). Although it has been reported that coconut shell aggregate reduces strength due to the poor bond between CS aggregate and cement paste resulting in porous concrete which increases water absorption13. Fibers behave as crack arresters and don’t prevent cracks which prevent cracks from propagation in the concrete leading to more strong concrete resulting in less water absorption33. Furthermore, silica fume enhances density due to pozzolanic reaction i.e. giving secondary CSH gel which enhances the paste viscosity that results in more dense concrete which also contributes to decreasing water absorption. However, mix 4 (60% CS aggregate, 2.0% glass fibers, and 20% silica fume), shows more water absorption than the reference sample. the porosity of coconut shell aggregate is more than coarse aggregate, and if we add more quantity of coconut shell aggregate it will absorb more water than it should, and surplus water in the concrete is not good and it leads to multiple issues like honeycombing in concrete, porous concrete, and concrete with less strength and durability, hence it is recommended to utilized optimum quantity (45%) of coconut shell aggregate in concrete to make concrete suitable for structural application and eco-friendly.
Figure 12Water absorption of concrete at 7 and 28 days of curing.
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Strong acids can be found in different varieties, for example, nitric acid, hydrochloric acid, acetic acid, and sulfuric acid (H2SO4), etc. In this study, Sulfuric acid was taken as an acid strike, on a concrete sample with different dosages. The test outcome after exposure to acid is shown in terms of mass loss because of the H2SO4 attack on the specimens after 7 and 28 days for each blend as shown in Fig. 13. It can be noted that weight loss due to sulfuric acid considerably decreases up to Mix 3 (45% CS aggregate, 1.5% glass fibers, and 15% silica fume) and then increase gradually having maximum loss at Mix 3 Mix 4 (60% CS aggregate, 2.0% glass fibers, and 20% silica fume). Erosion of concrete is the dissolution of calcium aluminate and calcium hydroxide due to sulfuric acid35,37,38. Erosion speed will largely depend on sulfuric acid penetration rate into the concrete body and to reach calcium aluminate and calcium hydroxide. Therefore, improvement in the concrete porosity leads to increased density of concrete because of the addition of glass fibers. The rise in density would lead to less penetration rate of sulfuric acid in concrete. Although it has been reported that coconut shell aggregate reduces strength because of the poor bond between CS aggregate and cement paste resulting in porous concrete which decreases density13. Fibers behave as crack stoppers and not as cracks prevention which decreases void in hardening concrete leading to more dense concrete resulting in more dense concrete21,33. However, mix 4 (60% CS aggregate, 2.0% glass fibers, and 20% silica fume), shows more weight loss than the control/ blank mix. It is because at higher dosage (60% CS aggregate, 2.0% glass fibers, and 20% silica fume), the workability of concrete decreases which increases compaction efforts resulting porous concrete leading to less density which ultimately increases loss of the weight.
Figure 13Acid resistance of concrete at 7 and 28 days in acid.
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